Zack Polanski has said two-party politics is “dead” as the Greens won two mayoral contests in London and gained a swathe of council seats in local elections.
Hackney and Lewisham elected Green mayors, marking the first directly elected mayors for the party and ousting Labour in both London boroughs.
The Greens also took control of two councils, Waltham Forest in London and Norwich, and won constituency seats in the Scottish and Welsh parliaments.
Green Party leader Mr Polanski called it “a historic victory” when Zoe Garbett ousted Labour from the Hackney mayoralty on Friday, a position it had held since it was created in 2002.
He said: “Two-party politics is not just dying, it is dead and it is buried.
“And actually, whether it’s here that Labour have been rejected, or whether we’re seeing around the country, it’s very clear that the new politics is the Green Party versus Reform.”
Ms Garbett won with 35,720 votes to former Labour mayor Caroline Woodley’s 26,685.
Green candidate Liam Shrivastava won the mayoralty in Lewisham.
He received 35,265 votes as Labour’s Amanda De Ryk came second with 30,374 votes, with Reform UK’s Pete Newman third on 7,288.
The Greens took control of Norwich City Council, which had previously had no party with overall control.
The Labour leader of Camden Council, the borough that takes in Prime Minister Sir Keir Starmer’s Holborn and St Pancras constituency, lost his seat to the Greens.
Welsh Green leader Anthony Slaughter won one of six seats at Caerdydd Penarth to bring the party its first ever seat in the Senedd.
In Scotland, the party won seats in Edinburgh Central and Glasgow Southside.
Mr Polanski has called for Sir Keir to stand down, saying the results from local elections so far show that the country has rejected the Labour leader.
“My message to Keir Starmer is it’s time to go,” he said.
“The country has clearly rejected you. I think he’d been rejected before these elections even started.
“We had 14 years of Conservative austerity; Keir Starmer was voted in on a promise of change and, actually, what we’ve seen is very little change and in many ways things have got worse.”
The Greens have seen a surge in support since Mr Polanski took over as leader last year.
Ipsos political director Keiran Pedley noted that even in areas where Reform gains seats from Labour, this is sometimes down to Labour support being diverted to the Green Party.
“Sometimes it’s because of Labour support going to the Greens and them losing votes that way, then Reform sort of comes through the middle,” he said.
Read MoreElections results in full: Live map for every seat across England, Wales and Scotland
Local election results live: Wales first minister loses seat in Labour humiliation
Keir Starmer ‘not going to walk away’ despite Labour election woes
Farage tears apart Starmer’s red wall brick by brick as Labour heartlands decimated
Ministers rally round Starmer as electoral mauling reignites leadership rumblings
Third British national has suspected hantavirus linked to cruise ship
